In an elevator system, buffers are devices which are designed to stop a descending cab or counterweight that moves downwardly beyond its normal limit of travel. The buffers must be operable to produce an average retardation of 32.2 feet/sec/sec of the speed of the cab or counterweight. Elevator pit buffers are commonly spring buffers or oil buffers, the former being typically used for elevator speeds of up to 200 feet/min. and the latter for speeds above 200 feet/min.
It is readily apparent that the ability of the buffer to properly decelerate the cab or counterweight is difficult as elevator operating speed increases, and that the ultra high speed elevators (above 1800 feet/min.) which are highly desirable in high-rise buildings, require excessively long buffer pistons in order to operate properly. For example, an elevator operating speed of 2,500 feet/min. would require a 428-inch stroke in order to meet the 32.2 feet/sec/sec retardation target referred to above, and a 3,000 feet/min. elevator would require a 616-inch minimum stroke. Elevator codes allow the reduction of the minimum buffer stroke when an emergency terminal speed limiting device, which senses the car speed and automatically removes power from the driving machine motor and brake if the normal terminal stopping device fails to slow down the car at the terminal as intended, is used on the cab and/or counterweight; however, the reduced stroke cannot be less than one-third of the minimum strokes specified above. Thus, the 2,500 feet/min. system would require a 143-inch stroke, and the 3,000 feet/min. system would require a 205-inch stroke. These stroke requirements exceed the longest buffer strokes available in the elevator industry, which is approximately 84 inches.
If one were to design a conventional type oil buffer for a 205-inch stroke, the overall height of the buffer would be more than 40 feet, and the pit depth needed to accommodate such a buffer would be several feet more. This adds considerable expense to the cost of the building. In addition, the buffer itself would be substantially more expensive than a conventional buffer because of the larger piston diameter that would be needed to meet the slenderness ratio (piston length over radius of gyration of piston cross section) of the longer column. Code requires that this ratio shall not exceed 80. Considerable development work would also be needed to design and test such a large-size oil buffer.